We have completed during the past year development of methodology for the cytologic localization and identification of messenger RNA molecules of the "prevalent" class, i.e., present in copy numbers approximating 10 to the 3rd power/cell. Initial developments were based upon hybridization of 3H-poly U to the polyadenylate termini of all mRNAs in the poly A (plus) group. Further method development will be pursued with probes other than poly U, - in particular, with reverse transcribed complements to purified natural mRNAs known to be present in the population of interest. These include the several individual messages for histones and that for tubulin. All probes are being applied to sea urchin embryos, normal and centrifuged, intact and disaggregated to individual blastomere types (e.g., the micro-, meso-. and macromeres formed at the 16-cell stage). The ultimate purpose of this work is to explore the possibility that the so-called "maternal" messenger RNAs of eggs serve not only a general biochemical purpose, but are also asymmetrically distributed, whereby they become the determiners of morphogenetic potential. In parallel with these essentially cytological studies there will be group of separate programs of biochemical work, designed to provide critical background information on the patterns of protein synthesis in vivo and in vitro (directed by purified mRNA), the sequence representation and complexity of the mRNAs, and the variations of non-histone chromosomal proteins in eggs, zygotes, early cleavage stages, and cultured, disaggregated blastomeres from early cleavage of sea urchin embryos.